The present invention relates to an electric power converter that generates a DC output from a DC power supply or from an AC power supply. Specifically, the present invention relates to the soft switching function of an electric power converter capable of conducting two-way operations.
The circuit of a conventional electric power converter capable of conducting two-way operations is disclosed in the following Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-147475. The conventional circuit disclosed in the Patent Document 1 is shown in FIG. 3A.
The conventional circuit shown in FIG. 3A is described in connection with a single-phase AC power supply. The conventional circuit consists of a rectifier circuit including a diode bridge circuit having diodes 2 through 5, and a chopper circuit including reactor 21, diode 6, and switching device 15.
As switching device 15 is turned on, AC power supply 1 is short-circuited via the diode bridge circuit and reactor 21, energy is stored in reactor 21, and an AC input current increases.
Then, as switching device 15 is turned off, the energy stored in reactor 21 is fed via diode 6 to capacitor 33 and load 34, which constitute a DC output.
By controlling the ON and OFF of switching device 15, a rectified AC voltage (DC voltage) is converted to an arbitrary DC voltage. A soft switching circuit for the chopper circuit is configured by capacitor 31, diodes 7, 9, 10, voltage clamping element 30, transformer 22 and switching device 17.
FIG. 3B is a wave chart describing the operations of the circuit shown in FIG. 3A.
As switching device 17 is turned on, the current that circulates, during a period t1, from reactor 21 to reactor 21 via diode 6, capacitor 33, diode bridge circuit 40, and AC power supply 1 gradually changes the current path so as to circulate, due to the influence of the leakage inductance of transformer 22, from reactor 21 to reactor 21 via diode 7, primary winding 22a of transformer 22, switching device 17, diode bridge circuit 40, and AC power supply 1. Since the current that flows through switching device 17 increases gradually from zero during the commutation described above, switching device 17 performs soft switching at the turn-ON thereof.
Then, a period t2 starts. During the period t2, the current flowing through switching device 17 becomes equal to the current flowing through reactor 21 and diode 6 becomes OFF. Since the current flowing through diode 6 decreases gradually to zero, the surge voltage and the reverse recovery losses caused by the reverse recovery are reduced. At the same time, the electric charge stored in capacitor 31 (or in the parasitic capacitance of switching device 15) is discharged via a path connecting capacitor 31, diode 7, primary winding 22a of transformer 22, switching device 17, and capacitor 31. The electric charge stored in capacitor 31 is regenerated to the output side via secondary winding 22b of transformer 22 and diode 10.
By turning on switching device 15 after the voltage thereof lowers to zero in a period t3, a difference current, which is the difference between the current flowing through primary winding 22a of transformer 22 and the current flowing through reactor 21, flows through switching device 15. Since the difference current that flows through switching device 15 initially flows through parasitic diode 12, the current that flows through switching device 15 increases gradually from a negative value. Therefore, switching device 15 performs soft switching at the state of the turn-ON thereof.
Then, the current that has been circulating from reactor 21 to reactor 21 via diode 7, primary winding 22a of transformer 22, switching device 17, diode bridge circuit 40, and AC power supply 1 gradually changes so as to circulate from reactor 21 to reactor 21 via switching device 15, diode bridge circuit 40, and AC power supply 1. At the same time, the energy stored in the leakage inductance of transformer 22 is fed to the output side via secondary winding 22b of transformer 22 and diode 10. The current that flows through switching device 17 decreases gradually to zero. Since switching device 17 is brought into the OFF-state thereof after the current that flows through switching device 17 reaches zero, switching device 17 performs soft switching at the state of the turn-OFF thereof.
When switching device 15 is turned off, the voltage of switching device 15 rises gradually due to the current flowing through capacitor 31. Therefore, the turn-OFF losses are reduced. Thus, switching devices 15 and 17 perform soft switching.
In a period t4, a reset voltage equal to the voltage clamped by voltage clamping element 30 is caused across primary winding 22a of transformer 22. A voltage, which is as high as the product of the reset voltage and the winding ratio of transformer 22, is generated across secondary winding 22b of transformer 22. The sum of the DC output voltage and the voltage across secondary winding 22b of transformer 22 is applied to diode 10. By setting the clamping voltage of voltage clamping element 30 to be low, the voltage applied to diode 10 is reduced.
FIG. 4A is a circuit diagram of another conventional electric power converter disclosed in the Patent Document 1.
In FIG. 4A, a rectifier circuit is configured by reactor 21, diodes 2 through 5, and switching devices 15 and 16. Switching device 15 and capacitor 31 are connected in parallel to diode 3. Switching device 16 and capacitor 32 are connected in parallel to diode 5. AC power supply 1 is connected between the series connection point of diodes 2 and 3 and the series connection point of diodes 4 and 5 via reactor 21. Capacitor 33 and load 34 are connected between the DC terminals of the diode bridge circuit.
The parasitic diode of switching device 15 may be used in substitution for diode 3. The parasitic diode of switching device 16 may be used in substitution for diode 5. The soft switching circuit for the rectifier circuit is configured by diodes 7 through 10, switching device 17, transformer 20, and voltage clamping element 30.
FIG. 4B is a wave chart describing the operations of the circuit shown in FIG. 4A.
As switching device 15 is turned on when the AC power supply voltage is positive, the AC input current, circulating from AC power supply 1 to AC power supply 1 via reactor 21, switching device 15, and diode 5, increases while storing energy in reactor 21. Then, as switching device 15 is turned off, the energy stored in reactor 21 is fed to the DC output side via a path connecting reactor 21, diode 2, capacitor 33, diode 5, AC power supply 1 and reactor 21. Therefore, it is possible to convert an AC power supply voltage to an arbitrary DC voltage by controlling the ON and OFF of switching device 15 when the AC power supply voltage is positive. In the same manner, it is possible to convert an AC power supply voltage to an arbitrary DC voltage by controlling the ON and OFF of switching device 16 when the AC power supply voltage is negative.
In FIG. 4A, diodes 7 and 8 are disposed in substitution for diode 7 in FIG. 3A. In FIG. 4A, diode 8 works for diode 7 in FIG. 3A, when the AC power supply voltage is positive. Diode 7 works for diode 7 in FIG. 3A, when the AC power supply voltage is negative. Since switching device 15 is turned on and off when the AC power supply voltage is positive, the electric charge stored in capacitor 31 is regenerated to the DC output side through the operations similar to the operations conducted in the circuit shown in FIG. 3A. Since a current always flows through diode 5 when the AC power supply voltage is positive, capacitor 32 stores no electric charge.
When the AC power supply voltage is negative, the electric charge stored in capacitor 32 is regenerated to the load side through the operations similar to the operations conducted in the circuit shown in FIG. 3A. Therefore, the circuit shown in FIG. 4A conducts operations similar to the operations conducted by the circuit shown in FIG. 3A. Switching devices 15, 16, and 17 and diodes 2 and 4 conduct soft switching. Since the sum of the DC output voltage and the secondary winding voltage of transformer 22 is applied to diode 10 in the circuit shown in FIG. 4A in the same manner as in FIG. 3A, the voltage applied to diode 10 is reduced by setting the clamping voltage of voltage clamping element 30 to be low.
For performing two-way electric power conversion, Patent Document 2: Japanese Unexamined Patent Application Publication No. Sho 64 (1989)-064557 discloses a combination of a buck chopper and a boost chopper. For the boost chopper, a boost chopper including an auxiliary chopper and disclosed in Patent Document 3: Japanese Unexamined Patent Application Publication No. Hei 05 (1993)-328714 may be used. However, the boost chopper including an auxiliary chopper and disclosed in the Patent Document 3 includes many circuit component parts. Moreover, the boost chopper including an auxiliary chopper and disclosed in the Patent Document 3 is large in size and expensive.
For realizing two-way electric power conversion in the conventional circuit shown in FIG. 3A, it is necessary to replace diode 6 by a switching device. For realizing two-way electric power conversion in the conventional circuit shown in FIG. 4A, it is necessary to replace diodes 2 and 4 by switching devices. The replacing switching device or the replacing switching devices can not perform soft switching.
In view of the foregoing, it would be desirable to obviate the problems described above, and to provide a two-way electric power converter that facilitates conducting soft switching operations inexpensively with low conversion losses.
Further objects and advantages of the invention will be apparent from the following description of the invention.